I. Field of the Invention
The present invention relates to wireless communications, and particularly to signal reception.
II. Description of the Related Art
A wireless receiver may receive a signal that has traveled several distinct propagation paths as generated by the multipath characteristics of the transmission channel. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In a mobile radio channel of a system for cellular communications, the multipath may be created by the reflections of the signal from obstacles in the environment, such as buildings, trees, cars, and people. In general, the mobile radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time location, attenuation, and phase as a function of the time that the ideal impulse was transmitted.
The multipath characteristic of a channel can result in signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of xcex4 with a phase shift of xcex8 radians, and the second path has an attenuation factor of X dB, a time delay of xcex4 with a phase shift of xcex8+xcfx80 radians, no signal would be received at the output of the channel.
In a code-division multiple-access (CDMA) cellular telephone system, a common frequency band is used, allowing for simultaneous communication between a mobile station (e.g. a cellular telephone, wireless local loop station, or wireless modem) and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high-speed pseudonoise (PN) code. The PN code is used to modulate signals transmitted between the base stations and the mobile stations. Transmitter stations using different PN codes, or the same PN code offset in time, produce signals that may be distinguished at the receiving station. The PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several distinct propagation paths.
In narrow-band modulation systems such as the analog FM modulation employed by conventional radio telephone systems, the existence of multiple paths in the radio channel may result in severe multipath fading. In a wideband CDMA system, however, instances of a transmitted signal that are received over different paths may be discriminated in the demodulation process. This discrimination not only greatly reduces the severity of multipath fading but also provides an advantage to the CDMA system.
The deleterious effects of fading can be mitigated by controlling transmitter power in the CDMA system. A system for base station and mobile station power control is disclosed in U.S. Pat. No. 5,056,109 entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,xe2x80x9d issued Oct. 8, 1991, and assigned to the Assignee of the present invention. Furthermore the effect of multipath fading can be reduced through communication with multiple base stations using a soft handoff process. A handoff process is disclosed in U.S. Pat. No. 5,101,501 entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d issued Oct. 8, 1991, and assigned to the Assignee of the present invention.
The existence of multipath can provide path diversity to a wideband spread spectrum system. A spread spectrum system generates a spread information signal by modulating an information signal with a pseudonoise (PN) code. Generally the PN code runs at many times the rate of the information signal. The rate at which the PN code is generated is called the chip rate, and the duration of one data bit of the PN code is called the chip time. If two or more paths are available that have a differential path delay greater than one chip time, then two or more processing elements (called demodulation elements or fingers) may be employed to separately demodulate the signal instances received over those paths. Such signals typically exhibit independence in multipath fadingxe2x80x94i.e., they do not usually fade togetherxe2x80x94so that loss of signal occurs only when all of the signal instances being demodulated experience a fade at the same time. Therefore, the output of the two or more demodulation elements can be combined to obtain path diversity. In an ideal system, both the base station and the mobile station employ multiple demodulation elements.
As a mobile station moves through the physical environment, the number and qualities of the signal paths may vary constantly, both as received at the mobile station and as received at the base station. Therefore, a receiver may use a special processing element (called a searcher element) that continually scans the channel in the time domain to determine the existence, time offset, and the signal strength of signals in the multiple path environment. The output of the searcher element provides information that may be used to ensure that the demodulation elements are tracking the most advantageous paths.
In an exemplary CDMA cellular telephone system, each base station transmits a pilot signal that is used by the mobile stations to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the base station transmitted signals. The pilot signals transmitted by the various base stations in a system may all use the same PN code, with each base station""s signal being spread using a different code phase offset (i.e. the PN codes transmitted by neighboring base stations are identical but skewed in time with respect to one another). This code phase offset allows the pilot signals to be distinguished from one another according to the base station from which they originate. A typical base station configuration may also contain multiple sectors, with each sector comprising independent transmit and receive antennas and having its own PN code or code offset. While the mobile station is in the call inactive mode, its searcher element may continue to scan the received signal at the code offsets corresponding to the transmitted pilot signals of nearby base stations. When a call is initiated, a PN code address is determined for use during this call. The code address may be either assigned by the base station or determined by prearrangement based upon the identity of the mobile station. After a call is initiated, the mobile station""s searcher element continues to scan the pilot signals transmitted by neighboring base stations, and a mobile station may communicate with several base stations at once.
Diversity combining in the mobile station significantly advances the quality and reliability of communications in a cellular telephone system. Diversity exploits the random nature of radio propagation by finding independent (or at least highly uncorrelated) signal paths for communication. If one radio path undergoes a deep fade, the receiver attains another independent path having a stronger signal. By having more than one path to select from, the signal-to-noise ratio at the receiver may be improved. A form of maximal ratio combining may be used to increase the benefit in which the signal-to-noise ratio is determined for each path. Each path may then be combined with the contributions from the other paths weighted according to the signal-to-noise ratio. Combining may be coherent because pilot signal demodulation allows the phase of each path to be determined.
A description of a three-step method for assigning multiple demodulation elements in a mobile station of a spread spectrum system follows. This description may also be found in U.S. Pat. No. 5,490,165 (xe2x80x9cthe ""165 patentxe2x80x9d), issued Feb. 6, 1996, and assigned to the assignee of the present invention. In the first step, a searcher element within the mobile station performs a survey in which it scans a window of time offsets around a nominal arrival time of each signal of each base station with which active communication has been established. Each survey yields a list of survey paths that comprises pilot signal strengths, time offsets, and a corresponding base station pilot offset. The searcher element passes the information to a controller, which attempts to match the time offset of each survey path to the time offset of the paths currently being demodulated by the demodulation elements. If multiple demodulation paths match one survey path, then only the demodulation element having the strongest signal strength indication remains assigned to that path, and the other elements assigned to the survey path are labeled xe2x80x9cfree.xe2x80x9d If a path currently being demodulated does not correspond to any survey path, then a survey path entry based on the demodulation path information is added to the list of survey paths.
In the second step, the controller considers the survey paths in decreasing order of signal strength (i.e. with the survey path having the strongest signal strength being considered first). If, for the survey path under consideration, no demodulation element is assigned to any path in the corresponding sector, the controller attempts to assign a demodulation element to the survey path according to the following procedure. If any demodulation elements are currently unassigned or labeled xe2x80x9cfree,xe2x80x9d one of these demodulation elements is assigned to the survey path. If no such demodulation element is found, the demodulation element having the weakest path that is not the only demodulation path from its base station sector is re-assigned to the survey path. If no such demodulation element is found, a demodulation element assigned to the weakest path is re-assigned to the survey path if the survey path""s signal strength is at least 3 dB stronger than the signal strength of the weakest demodulation path. This process continues until a re-assignment occurs or until the entire survey path list is traversed with no reassignments.
In the third step, if no reassignments have occurred in the second step, then the controller considers the survey paths again in order of decreasing signal strength. If no demodulation element is assigned to the survey path, the controller may assign an unassigned or free demodulation element to the path. If no such demodulation element is found, the controller may re-assign a demodulation element that is assigned to the same base station sector as the survey path if the survey path is more than 3 dB stronger than the current demodulation path. Alternatively, the controller may re-assign the weakest demodulation element that is assigned to any base station sector having two or more assigned demodulation elements if the survey path is more than 3 dB stronger than the demodulation path. Once a re-assignment occurs or the procedure fails an attempt to reassign a demodulation element to the survey path under consideration, the method begins again.
A method of reassignment as described above may require two traversals through the path list to obtain a desired level of diversity. It is desirable to obtain a reassignment of tracking elements (e.g. demodulation elements) that achieves a desired level of diversity but does not require two traversals of the path list. For example, it may be desirable to obtain a reassignment of tracking elements that achieves a desired level of path diversity, as well as a desired level of sector or cell diversity, but does not require two traversals of the path list.
Embodiments of the present invention provide a method of assigning tracking elements including sorting a list of potential assignments (e.g. signal instances within a received signal) according to a diversity criterion. In certain embodiments, a list of signal instances is sorted first according to a measure of energy, then again according to a diversity criterion such as a predetermined level of cell diversity. In such embodiments, an order of at least a portion of the sorted list may be biased in favor of the diversity criterion. For example, the list may be sorted such that each of the first n potential assignments on the list corresponds to a different source. An aspect of additional embodiments of the invention is that a tracking element that outputs demodulated symbols when in a locked state may be labeled as available for assignment when in the locked state.
Other embodiments of the present invention provide a tracking stage of a mobile station that includes a searcher element, a set of tracking elements, and a controller. The controller sorts a list of potential assignments, as collected (at least initially) by the searcher element, according to a diversity criterion and modifies a current assignment of at least one of the tracking elements according to the sorted list.
Other embodiments of the present invention provide a data storage medium (e.g. a magnetic or optical disk or other recording, impression, or fixation; a semiconductor or ferromagnetic device such as a memory or integrated circuit; a printed code listing) having machine-readable code including instructions executable by an array of logic elements, where the instructions provide a method of assigning tracking elements including sorting a list of potential assignments according to a diversity criterion.